1. Field of the Invention
The present invention relates generally to an optical cross-connecting device. More particularly, the invention relates to an optical cross-connecting device performing switching of a plurality of wavelength multiplexed signals and adding/dropping of the signal.
2. Description of the Related Art
FIG. 12 is a block diagram showing a construction of the typical conventional optical cross-connecting device. In FIG. 12, a plurality of the wavelength multiplexed signals input from a plurality of optical fibers 101 of a plurality of transmission paths is divided into individual unity wavelength signals by respective of corresponding wavelength demultiplexers 102. In this case, for example, respective of a plurality of wavelength multiplexed signals are respectively multiplexed signals of a hundred sixty unit wavelength signals respectively having wavelength λ1 to λ160. Each optical demultiplexer 102 has a function for dividing the corresponding wavelength multiplexed signal into hundred sixty unity wavelength signals.
Then, the wavelength signals divided by a plurality of optical demultiplexers 102 are subject to switching per wavelength signal by an optical switch 103 and to adding/dropping process of the signal (termination of the signal), per respective wavelength λ1 to λ160. Then, again, the wavelength signals are multiplexed by respective of optical multiplexers 104 into a plurality of wavelength multiplexed signals to be output from optical fibers 105 of a plurality of transmission paths.
The foregoing system is constructed for enabling switching of all wavelength signals to inherently make scale of switch huge to cause difficulty in realization in technical and economical point of view. Also, when large proportion of wavelength signals pass through as they are and only quite limited number of signals are to be switched and added/dropped, such system is quite inefficient.
Next, when the wavelength band to be used becomes high density and wide range, in view point of characteristics of transmission and difficulty in production of the wavelength demultiplexers and wavelength multiplexers adapted for the high density and wide range and for other reason, the filtering and multiplexing method of the wavelength to demultiplex or multiplex all of the wavelength multiplexed signals in the lump like as 102 and 104 as shown in FIG. 12 is less practical. Instead, as shown in FIG. 13, a demultiplexing method to once divide the wavelength multiplexed signal into a plurality of wavelength groups by a wavelength group demultiplexer 111, and then divide each wavelength group into individual wavelength signals by wavelength demultiplexers 112-1 to 112-U, may be employed. And a multiplexing method to once multiplex the individual wavelength signals into a plurality of wavelength groups by wavelength multiplexers 113-1 to 113-U and then multiplex a plurality of the wavelength groups into the wavelength multiplexed signal by a wavelength group multiplexer, may be employed.
As a method for forming the wavelength group various methods, such as a method to aggregate wavelength signals having close wavelengths into the same group as shown in FIG. 14A, a method to aggregate the wavelength signals having the same period relative to a wavelength axis into the wavelength group as shown in FIG. 14B, a method to combine the methods in FIGS. 14A and 14B, and other method, may be considered. (In FIGS. 14A, 14B and 14C, the same pattern means belonging in the same wavelength group.)
FIG. 15 shows an example of the case of forming the wavelength group from a band which can be amplified by an optical amplifier, as forming method of the wavelength group. The shown example is directed to the case where the amplifier capable of amplifying all bands of a hundred sixty waves having wavelengths of λ1 to λ160 cannot be produced. In such case, the wavelength multiplexed signal containing wavelengths λ1 to λ160 is divided into a wavelength group of eighty waves having wavelengths λ1 to λ80 and a wavelength group of eighty waves having wavelengths λ81 to λ160 to have two wavelength groups, by the wavelength demultiplexer 111. It should be noted that amplifiers 121 and 122 are optical amplifiers having foregoing bands to be amplified. An example forming the wavelength group from the characteristics of the amplifier shown in FIG. 15 is the example illustrated in FIG. 14A.
On the other hand, FIG. 16 shows the case where the wavelength group is formed from the characteristics of the wavelength demultiplexer or wavelength multiplexer. When all intervals of each wavelengths λ1 to λ160 are 0.4 nm as shown in FIG. 16B, a wavelength multiplexer multiplexing a light beam having wavelengths λ1 to λ160 obtained from transmitters 130-1 to 130-160 in a lump, is technically difficult. Namely, as the wavelength multiplexer 131, a filter having band pass characteristics permitting respective wavelengths λ1 to λ160 at interval of 0.4 nm has to be used as shown in FIG. 14B.
However, in practice, filter having such band pass characteristics is difficult to realize. Therefore, as shown in FIG. 16C, two filters having pass band of 0.8 nm interval are used so that one filter passes odd number order of wavelengths λ1, λ3, . . . , and the other filter passes even number order of wavelengths λ2, λ4, . . . to form the wavelength group. This example is the example illustrated in FIG. 14B.
In case of the construction as shown in FIG. 13, the wavelength signal passes through multiple stages of wavelength demultiplexers/multiplexers or through wavelength group demultiplexers/multiplexers, as one kind of optical filter, to cut spectrum component to increase signal degradation.
FIG. 17 is a block diagram showing another construction of the typical conventional optical cross-connecting device. Wave lengths of respective of individual wavelength signals demultiplexed by wavelength demuliplexers 102 are converted into the same wavelength (e.g. λ0) by respective wavelength converters 121. These wavelength signals of the same wavelength are input to an optical switch 122 to be switched per individual signal or added/dropped the signal therein. The individual signals output from the optical switch 122 are input to each wavelength converters 123 to be converted into each individual wavelength signals by the wavelength converters 123. (It should be noted that, in the wavelength converter, certain wavelength signal is once converted into an electric signal and then converted into the wavelength signal having wavelength λ0.) The individual wavelength signals output from the wavelength converters 123 are multiplexed by the optical wavelength multiplexers 104 and output from optical fibers 105 of a plurality of transmission paths.
It should be noted that, in FIG. 17, when 121 are optical receivers, each individual electric signal photoelectric converted by the optical receivers 121 are input to the electrical switch 122. By the electrical switch 122, switching per individual electrical signal or add/drop of the signal are performed. The individual signals output from the electrical switch 122 are input to respective optical transmitters 123, and converted into individual wavelength signals by the optical transmitters 123. The individual wavelength signals output from the optical transmitters 123 are multiplexed by the optical wavelength multiplexers 104 to be output from optical fibers 105 of a plurality of transmission paths.
In such construction, the optical signals input from the optical fibers 101 of a plurality of transmission paths are once converted into the electrical signals (as set forth above, even in wavelength conversion process, the signals are once converted). Therefore, signal degradation by passing a plurality of stages of filters can be restricted. However, similarly to the construction of FIG. 12, scale of the switch is large to cause technical and economical difficulty in practicing. Since the wavelength converters and the optical transmitters/receivers are required for all signals, scale and cost for the system can be further increased.
On the other hand, when large proportion of wavelength signals pass through as they are and only quite limited number of signals are to be switched and added/dropped, such system is quite inefficient similarly to the construction shown in FIG. 12. Namely, in case of switching equipment in small or middle size city, large proportion of signals are simply relayed to the switching equipment in the next city, and terminal process, such as switching or adding/dropping for such signals is not required. Despite of this fact, conversion of the signal per wavelength is performed for all signals.
As set forth above, the conventional optical cross-connecting device is large in scale of the switch for demultiplexing and multiplexing the overall wavelengths to be technically and economically difficult to realize. Also, when large proportion of wavelength signals pass through as they are and only quite limited number of signals are to be switched and added/dropped, such as in the case of the switching equipment in small and medium size city, such system is quite inefficient. Further problem is also encountered in signal degradation by passing the optical signal through plurality of stages of wavelength group demultiplexer and multiplexer or wavelength demultiplexer/multiplexer, and the scale and cost of the system are increased by employment of the wavelength converters and optical transmitters/receivers for all of the wavelength signals.